Operational efficiency and the overall output of a gas turbine engine generally increases as the temperature of the hot combustion gas stream increases. Higher combustion gas stream temperatures, however, may produce higher levels of nitrogen oxides (NOX) and other types of regulated emissions. A balancing act thus exists between the benefits of operating the gas turbine engine in an efficient high temperature range while also ensuring that the output of nitrogen oxides and other types of regulated emissions remain below mandated levels. Moreover, varying load levels, varying ambient conditions, and many other types of operational parameters and design requirements also may have a significant impact on overall gas turbine engine efficiency and emissions.
Lower emission levels of nitrogen oxides and the like may be promoted by mixing the feel stream and the air stream prior to combustion. Such premixing tends to reduce combustion temperature gradients and the output of nitrogen oxides. Certain combustors may include a premixer positioned upstream of a combustion zone and configured to mix at least portions of the fuel stream and the air stream prior to combustion. According to one known premixer configuration, a combustor may include a micro-mixer having an array of small tubes arranged within a plenum such that each tube mixes small volumes of the fuel stream and the air stream upstream of the combustion zone.
In many gas turbine engine applications, it may be desirable to have a combustor that is capable of operating on either gas fuel, such as natural gas or syngas, or liquid fuel, such as diesel fuel, kerosene, ethanol, or a water and oil mixture. Such fuel flexibility, however, often requires complex and costly feel injection systems that may sacrifice operability or performance when operating on one type of fuel or the other. Moreover, adapting such fuel injection systems to operate in conjunction with a premixer, such as a micro-mixer, may present substantial challenges in fuel injection as well as maintaining nitrogen oxides and other types of regulated emissions below mandated levels. Because liquid fuels may be about fifty times denser than gas feels, the injection ports and fuel delivery networks required to inject liquid fuel into each tube of the micro-mixer would need to be much smaller and more complex than those typically used to inject gas fuel in a similar manner. However, because liquid fuels are prone to thermal breakdown or coking within fuel passages at higher temperatures (e.g., about 290° F.), the fuel delivery networks would likely coke shut after only a few minutes of delivering liquid fuel to the micro-mixer.
Certain dual fuel combustors may be configured to inject gas fuel in the combustor during one mode of operation, and to inject and vaporize liquid fuel in the combustor during another mode of operation. Such injection and vaporization of the liquid fuel, however, may result in increased risk of auto-ignition, carbon formation, flashback, and flame holding at the head end of the combustor. According to one known combustor configuration, modifications to the injection system to address these risks may negatively impact the ability to burn gas fuel with acceptable operability. According to another known combustor configuration, the liquid, fuel may be vaporized outside of the combustor in an auxiliary vapor production system and then injected into the combustor through the gas fuel injection system. The vapor production system may require large quantities of an inert gas, such as nitrogen, and ultimately may increase parasitic loads, complexity, and cost of the overall gas turbine engine. Other dual fuel combustors may be configured to inject fuel into a secondary combustion stage and thus would not be compatible with a primary fuel premixer, such as a micro-mixer, in a primary combustion stage. Accordingly, such combustors may present challenges in maintaining nitrogen oxides and other types of regulated emissions below mandated levels. Still other dual fuel combustors may be configured to vaporize liquid fuel within the combustion zone, which also would not be compatible with a primary fuel premixer and may present challenges in emissions control.
There is thus a desire for an improved dual fuel combustor configured to inject gas fuel in the combustor during one mode of operation, and to inject and vaporize liquid fuel in the combustor during another mode of operation. Specifically, such a combustor should address the risks of auto-ignition, carbon formation, flashback, and flame holding, while providing acceptable operability when burning gas fuel or liquid fuel. Further, such a combustor should include a fuel injection system that is compatible with a primary fuel premixer and maintains regulated emissions below mandated levels, while also minimizing cost and complexity of the overall gas turbine engine.